Peak cleaning method

Poling and heating profiles used to measure thermally stimulated current. (1) Standard measurement, (11) partial heating method or peak cleaning method (111) thermal sampling method... 

However, in polymer systems there can be many internal relaxation modes and it is unreasonable to assume that a single relaxation process is responsible for the complex TSC curves typically recorded. In order to deconvolute complex TSC spectra into individual relaxation processes, where the Debye and Arrhenius relations are more applicable, two approaches are used. The first approach is called the partial heating method or peak cleaning method (Figure 6.29(H)). Following quenching and extinction of the applied electric field, the TSC curve of the polarized sample is measured as the sample is heated at a controlled rate to. The sample is requenched from to Tq, and the TSC curve is subsequently... 

Figure 2.6 Application of thermal cleaning method of TSDC peaks to Aso.01Seo.99. Curve 1—initial TSDC curve. The temperature in the successive cycles of heating is 166 K (2), 180 K (3), and 199 K (4).

The second cleaning method, specific to TSDC measurements and which may be apphed in our case, is due to Bucci et al. [23]. It consists of first polarizing the material at a temperature T, such that T 2 and removing the field at temperature Td such that [Pg.34]

The thermal cleaning method and partial heating are known as experimental separation methods of composite TSC spectrum and only initial rising part of TSC spectrum was used for estimation of Et value. The initial rise method(Garlick Gibson,1948) is the only one procedure to apply to data without a peak until now. However, in AEM-o, an application is possible to an omniformity-shaped TSG spectrum. 

Lamparczyk (1992) provided detailed methods for the sample preparation and TLC determination of fecal bile acids (cholic, chenodeoxycholic, deoxy-cholic, lithocholic, and ursideoxycholic acids). After the sample is cleaned up by liquid-liquid extraction, from 2 to 20 a1 of purified fecal bile solution (no more than 20 p,g of bile acids) is spotted in the preadsorbent zone of a 20 X 20-cm silica gel plate. The plate is developed in isooctane-2-propanol-acetic acid (30 10 1) for 40 min, dried, and developed again with isooctane-ethyl acetate-acetic acid (10 10 2) for 65 min. For quantitative analysis, the plate is dipped for 2 sec into a 0.2%, 2,7-dichlorofluorscein ethanol solution. Bile acid fluorescence is measured with a TLC scanner and the results calculated by the peak-height method. Rivas-Nass (1994) evaluated the influence of temperature, ionic strength, mobile-phase pH, and addition of modifiers to the solvent system on the silica gel TLC of bile acids. 

A powerful tool now employed is that of diode array detection (DAD). This function allows peaks detected by UV to be scanned, and provides a spectral profile for each suspected microcystin. Microcystins have characteristic absorption profiles in the wavelength range 200-300 nm, and these can be used as an indication of identity without the concomitant use of purified microcystin standards for all variants. A HPLC-DAD analytical method has also been devised for measurement of intracellular and extracellular microcystins in water samples containing cyanobacteria. This method involves filtration of the cyanobacteria from the water sample. The cyanobacterial cells present on the filter are extracted with methanol and analysed by HPLC. The filtered water is subjected to solid-phase clean-up using C g cartridges, before elution with methanol and then HPLC analysis. 

Figure 13.10 LC-LC chromatogram of a surface water sample spiked at 2 p.g 1 with ati azine, and its metabolites (registered at 220 nm). Conditions volume of sample injected, 2 ml clean-up time, 2.60 min ti ansfer time, 4.2 min The blank was subtracted. Peak identification is as follows 1, DIA 2, HA 3, DEA 4, atrazine. Reprinted from Journal of Chromatography, A 778, F. Hernandez et al, New method for the rapid detemiination of triazine herbicides and some of thek main metabolites in water by using coupled-column liquid cliromatography and large volume injection , pp. 171-181, copyright 1997, with permission from Elsevier Science.

A variety of formats and options for different types of applications are possible in CE, such as micellar electrokinetic chromatography (MEKC), isotachophoresis (ITP), and capillary gel electrophoresis (CGE). The main applications for CE concern biochemical applications, but CE can also be useful in pesticide methods. The main problem with CE for residue analysis of small molecules has been the low sensitivity of detection in the narrow capillary used in the separation. With the development of extended detection pathlengths and special optics, absorbance detection can give reasonably low detection limits in clean samples. However, complex samples can be very difficult to analyze using capillary electrophoresis/ultraviolet detection (CE/UV). CE with laser-induced fluorescence detection can provide an extraordinarily low LOQ, but the analytes must be fluorescent with excitation peaks at common laser wavelengths for this approach to work. Derivatization of the analytes with appropriate fluorescent labels may be possible, as is done in biochemical applications, but pesticide analysis has not been such an important application to utilize such an approach. 

Table 4.45 shows the main features of SEC. This technique has become an indispensable tool for polymer characterisation. SEC has some advantages over other LC methods, such as the predictability of the end of a chromatographic run and of the retention times in a calibrated chromatographic system. SEC is an attractive technique for prefractionation or sample clean-up prior to a more sensitive RPLC technique. This intermediate step is especially interesting for experimental purposes whenever polymer matrix interference cannot be separated from the peak of interest [647]. Disadvantages are that the whole separation must be eluted within the... 

Studies on the adsorption of hydrogen from the gas phase had provided strong evidence for the existenee of two forms of adsorbed hydrogen and the AC impedance studies were supported by the results of the new LSV and CV techniques. The early measurements using the voltammetry methods were hampered by the use of impure electrolytes which resulted in ill-defined hydrogen adsorption and desorption peaks but the realisation of the need for a clean electrochemical system soon resulted in the routine observation of the now familiar twin Hads peaks. 

Procedure Dissolve accurately 22.5 mg of /ram-clomiphene citrate and 52.5 mg of cis-clomiphene citrate (approx. 1 2.3) into 10 ml of DW in a clean 50 ml separating funnel. Add to it 1 ml solution of sodium hydroxide (5% w/v in DW). In the alkaline medium the base is liberated which is extracted successively with 3 portions of solvent ether (10 ml each). The combined ethereal layer is washed with two portions of DW (10 ml each). The resulting ethereal fraction is dried over anhydrous sodium sulphate, filter, evaporate to diyness carefully over an electric water-bath and dissolve the residue in 1 ml of CS2. Now, record the absorption curve in a 0.2 mm cell over the range 12.50 to 14.00 pm. Calculate the absorbance for the peaks at 13.16 and 13.51 pm respectively by employing the base-line method (see section 3. l. B in this chapter) between the minima at 12.66 and 13.89 pm. 

The principal trap parameter, the activation energy, can be easily calculated from a single TSDC experiment by means of some characteristic elements of the peak, such as its half-width, inflection point, or initial part of current rise. The most useful one and, in fact, the most frequently exploited, is nndonbtedly the initial rise method [20], because it is always easily applied to a previously cleaned peak. 

Figure 3 shows a comparison between the two methods. The ultrasonically cleaned resin has 10 more contaminant peaks than the Soxhlet-extracted resin. However, the ultrasonic procedure achieves almost the same results as Soxhlet extraction in one-fourth the elapsed time. 

The definition of the reactant coordinate used in the MC pushing method may be derived from a separation into participant and bystander degrees of freedom, or it may be arrived at intuitively or empirically. Generally speaking, the more cleanly a reaction coordinate separates the two peaks of a bimodal distribution, the higher the conversion coefficient that can be achieved with it. 

Method A 50 /jl (ca 250 ng) of the stock solution or of the purified extract is transferred into a reaction tube. One ml of boron trifluoride is added to the methanol, the cap is closed, and the tube is kept on a block heater (80°C) for 10 min. Two ml of water and 2 ml of -hexane are added. After thorough mixing, the layers are left to separate. Using disposable glass Pasteur pipettes, the upper layer is transferred to a clean, small vial. The extraction is repeated another two times with 2-ml portions of n-hexane. The hexane extracts are taken to dryness with a nitrogen stream, and the residue is dissolved in 1 -2 ml of injection solvent. Ochratoxin A is confirmed by the presence of an OA-methyl ester peak at delayed retention time and the disappearance of the OA peak (62). 

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